1. Field of the Invention
The present invention relates to an electrophotographic carrier and an electrophotographic developer using the electrophotographic carrier, and, in particular, to an electrophotographic carrier and an electrophotographic developer using the electrophotographic carrier, which are used in an image forming process.
2. Prior Art
In the past, a binary type developing method has been known as one electrostatic latent image developing method used in electrophotography. In the binary type developing method, an insulating non-magnetic toner is mixed with magnetic carrier particles to charge the toner by friction. The resulting developer is then transferred to contact an electrostatic latent image and thereby develop the electrostatic latent image.
The particulate carrier used in such a binary type developing method is generally formed by coating a magnetic carrier core material with an appropriate material with the view of preventing the toner from forming a film on the surface of the carrier, forming a carrier with a uniform surface, prolonging the life of the developer, and controlling damage to a sensitive material by the carrier and the charge amount.
However, conventional carriers have not satisfied the durability requirements since coatings of these tend to be peeled off by the impact caused by agitation and the like during operation.
In order to solve such a problem, the present inventors have developed and proposed a technology for forming a polyolefin type resin coating by polymerizing an olefin type monomer directly on particles of carrier core material such as ferrite or the like (for example, Japanese Patent Application Laid-open No. 187771/1990 and the like). The carrier provided with the polyolefin type resin coating prepared in this manner has excellent characteristics in which the adhesion strength between a core material and a coating is very high and hence the image quality never deteriorates even if the copying operation is continued for a long period of time. Also, the carrier provided with the polyolefin type resin coating has superior durability and spent resistance.
However, on the other hand, this carrier provided with the polyolefin type resin coating has the drawback that the charge polarity, charge amount, and the like can not be optionally controlled and there is no durability to necessarily satisfy resistance to a spent phenomenon caused by adhering of external additives.
In order to solve the above problems, Japanese Patent Application Laid-open No. 100242/1978 discloses a material produced by compounding nigrosine in a carrier coating resin to increase the negative charge and Japanese Patent Application Laid-open No. 9661/1986 discloses the instance in which the fluidity is improved by adding a fluidity improving agent. Also, Japanese Patent Application Laid-open No. 210365/1990 discloses a technology for homogenizing charge properties and preventing a spent phenomenon by adding any one of electroconductive particles, inorganic filling particles, and a charge control agent.
However, these technologies can not satisfy both optional controllability of the charge polarity and charge amount and prevention of a spent phenomenon caused by external additives for a toner while making use of the excellent characteristics of the carrier coated with a polyolefin type resin.
This invention has been achieved in view of this situation and has an object of providing an electrophotographic carrier which can satisfy both optional controllability of the charge polarity and charge amount and effective prevention of the spent phenomenon of external additives, while making use of the high durability of a carrier coated with a polyolefin type resin. The present invention has also an object to provide an electrophotographic developer using such an electrophotographic carrier.